Child properties with JDO and AppEngine - google-app-engine

Question

I have the following code:
#PersistenceCapable(identityType = IdentityType.APPLICATION, detachable="true")
public class A {
#Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
#PrimaryKey
private Key key;
#Persistent
private B b;
#Persistent
private int id;
// ...
}
#PersistenceCapable(identityType = IdentityType.APPLICATION, detachable="true")
public class B {
#Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
#PrimaryKey
private Key key;
#Persistent
private int id;
// ...
}
Now what I need to be able to do, is retrieve an instance of B, and refer to it from an instance of A like this:
B b = DAL.getBById(1);
A a = new A();
a.setB(b);
When I pass a to the makePersistent() method of the PersistenceManager, two things that I don't need happen:
1) a new instance of B is created
2) the reference A makes to b is null
Could someone tell me what I am doing wrong?
Thanks!

Answer 1

A field value can contain an instance of a Serializable class, storing the serialized value of the instance in a single property value of the type Blob. To tell JDO to serialize the value, the field uses the annotation #Persistent(serialized=true). Blob values are not indexed and cannot be used in query filters or sort orders.
Here is an example of a simple Serializable class that represents a file, including the file contents, a filename and a MIME type. This is not a JDO data class, so there are no persistence annotations.
import java.io.Serializable;
public class DownloadableFile implements Serializable {
private byte[] content;
private String filename;
private String mimeType;
// ... accessors ...
}To store an instance of a Serializable class as a Blob value in a property, declare a field whose type is the class, and use the #Persistent(serialized = "true") annotation:
import javax.jdo.annotations.Persistent;
import DownloadableFile;
// ...
#Persistent(serialized = "true")
private DownloadableFile file;
I your case you can use
import java.io.Serializable;
public class B implements Serializable {
private int xx;
....
..........
}
Then declare it in your data class
#Persistent(serialized = "true")
private B b;